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I have no physics education beyond high school, but I briefly became obsessed with the idea of efficient collimation of light. I eventually concluded that it mu

I have no physics education beyond high school, but I briefly became obsessed with the idea of efficient collimation of light. I eventually concluded that it must be impossible, because it would imply a global decrease in entropy, much like Maxwell's demon. But this article seems to suggest that it might still be possible. Have I misread it? Or maybe misinterpreted — e.g. maybe "efficient" would cover a system that does require energy input, and just doesn't require discarding a lot of the input light? Can anyone with more of a clue comment on this? I find the idea exciting because I can imagine endless practical uses for an efficient collimator.

mnw21cam 3y ago
It'd be balanced by the difficulty in getting the light into the material in the first place. You're right, you can't change the laws of physics. Light will only enter an object with lower refractive index if its angle of incidence is close enough to straight on that the beam coming through into the material has somewhere to go that satisfies Snell's law. When the object's refractive index is near zero, the object will only accept incoming light across an extremely narrow angle. This solves your concerns about having a global decrease in entropy. You can fudge this by making the incoming surface of the object match the shape of the incoming waves. Then all the light will be allowed in. However, that surface will only match the shape of the incoming waves for light originating from a tiny point in space. If your light source is larger than a tiny point in space, then the light coming from the larger area will bounce off the material and not enter it. In effect, the magic material will magically collimate a light source, but only if the light source is already fairly magic, and in that case a simple lens would suffice.
- jeffparsons 3y ago
  Thanks for the explanation. It makes me a little sad, but maybe it's for the best; efficient collimation of light would offer enormous destructive potential for $cheap, and that's not something the world needs any more of.
dekhn 3y ago
it sounds like etendue

mnw21cam 3y ago

It'd be balanced by the difficulty in getting the light into the material in the first place. You're right, you can't change the laws of physics. Light will only enter an object with lower refractive index if its angle of incidence is close enough to straight on that the beam coming through into the material has somewhere to go that satisfies Snell's law. When the object's refractive index is near zero, the object will only accept incoming light across an extremely narrow angle. This solves your concerns about having a global decrease in entropy. You can fudge this by making the incoming surface of the object match the shape of the incoming waves. Then all the light will be allowed in. However, that surface will only match the shape of the incoming waves for light originating from a tiny point in space. If your light source is larger than a tiny point in space, then the light coming from the larger area will bounce off the material and not enter it. In effect, the magic material will magically collimate a light source, but only if the light source is already fairly magic, and in that case a simple lens would suffice.

jeffparsons 3y ago
Thanks for the explanation. It makes me a little sad, but maybe it's for the best; efficient collimation of light would offer enormous destructive potential for $cheap, and that's not something the world needs any more of.

dekhn 3y ago

it sounds like etendue